T.O. 33B-1-1
6-7
6.2.2
Basic Requirements for Production of X-Rays.
X-rays are produced when some form of matter is struck by a rapidly moving electron. To accomplish this, three basic
requirements must be met.
6.2.2.1
Supply Of Electrons.
There must be a supply of the electrons. Fortunately, they can be supplied by simply raising the temperature of a
suitable material. An electron source is readily obtainable in as much as all matter is generally considered to be
composed of electrons and other minute particles. All that is necessary is to sufficiently heat the proper material. As
the temperature rises, the electrons become more and more agitated until finally they escape or "boil off” the material,
surrounding it in the form of an electron cloud (see Figure 6-5). This is known as thermionic emission. In an X-ray
tube the heated material is known as the filament, which is similar to the filament in a light bulb. Just as in a light bulb
the filament is heated by passing electrical current through it.
Figure 6-5. Electron Cloud.
6.2.2.1.1
This cloud of electrons simply hovers around and returns to the emitting substance unless some external action or force
pulls it away.
6.2.2.2
Movement of the Electrons.
Movement of the emitted electrons is the second step in producing X-rays. This movement is brought about by the
repelling and attracting forces inherent in electrical charges. The fundamental law of electrostatics states that like
charges repel each other and unlike charges attract each other. Electrons are negative charges, thus repel each other.
However, a stronger attracting force is needed to accelerate the electrons to a higher velocity. Therefore, a strong
opposite (positive) charge is used to move the electrons from one point to another. It is important that this movement is
conducted in a good vacuum, otherwise the electrons collide with air molecules and lose energy through ionization and
scattering. In an X-ray tube the anode is given a positive charge with respect to the filament, which is part of the
cathode.
